Many types of engineered structures, such as skyscrapers, bridges, and aircraft airframes, are subject to vibrational stresses which can be caused by aerodynamic forces due to wind, for example, or due to the airspeed of an aircraft in flight. The aerodynamic forces over such structures may cause an unstable oscillatory aeroelastic deformation, or vibration, of the structure referred to as flutter. Flutter may involve different types of motion, or stress, such as bending or twisting, combinations of which may be referred to as a mode (e.g., mode of deformation) or vibrational mode.
In the case of an aircraft wing in flight, for example, the aeroelastic deformations may be relatively mild and stable within the normal operating envelope of the aircraft. In the case of flutter, however, the aeroelastic deformations may be driven into an unstable mode in which the torsional (e.g., twisting) motion, for example, extracts energy from the airstream that drives the particular vibrational mode (e.g., the torsional motion itself or a combination of the torsional motion with some other motion such as bending) to increasingly higher amplitudes, causing oscillations of increasing amplitude that may result in damage to the wing.
To reduce the likelihood of such situations, exhaustive flight and wind tunnel tests are usually conducted to observe and record the flutter characteristics of the various aeroelastic structures of an aircraft over the entire flight envelope of the aircraft and to predict a safe operating speed and altitude envelope for the aircraft. In such testing, a number and variety of types of sensors, or transducers, may be attached to the various structures of the aircraft, such as the wing, fuselage and empennage. Sensors may include accelerometers, strain gauges, temperature sensors, and speed indicators, for example. During flight testing, various excitations may be provided for the sensors, such as deflecting a control surface, e.g., an aileron, elevator, rudder, or flap, for example, by some specific amount. Correspondence between the excitations provided (input) and response from the sensors (output) is included in the flight test data. The flight test data, to be useful, needs to be analyzed, typically by computer. Often times during flight, however, analysis results are needed before any further or new excitations may be provided, in order, for example, to avoid endangering the aircraft and flight crew. A worst case scenario is that the test flight is shut down until computation of analysis results can be completed, upon which test flights may be resumed. Such interruptions can be expensive in terms of aircraft downtime and inefficient use of facilities such as airstrips and control tower, ground, and flight crews. Thus, there is a need in the testing of structures for aeroelastic deformation and vibration response (e.g., flutter testing)—which can also have benefits for the engineering and testing of other wind-loaded structures or structures subject to induced vibration, such as buildings and bridges—for computationally efficient and fast methods of analyzing and using flutter test data.